Researchers at Inha University in South Korea have demonstrated that cellulose, the main ingredient in paper, can bend in response to electricity. The treated cellulose is lightweight, inexpensive, and has low power requirements, compared with similar electrically active materials.
The Korean researchers are now working with NASA to develop insect-sized, wirelessly powered flying vehicles with flapping paper wings. Such vehicles could fly into areas unsafe for humans and test for hazardous gases – or survey the surface of Mars from the air.
[Click here for images of this movable paper.]
The researchers, led by Jaehwan Kim, associate professor at the university, made the electrically active cellulose by dissolving paper pulp, forming it into sheets, and coating it with a layer of gold as an electrode. Some areas of the cellulose film are highly ordered, while in other areas, the cellulose strands are tangled like spaghetti. The movement of ions through the paper – and the movement of cellulose strands themselves, which have negative and positively charged ends – causes the paper to bend in response to an electrical current. The bending is driven by the ordered regions, but free space in disordered regions allows ions to flow more freely and adds to the paper’s ability to deform.
Materials that move in response to electrical current are called piezoelectrics. Kim’s cellulose is one of a new class of these materials, called electroactive polymers, that have generated excitement in the scientific community for their potential uses in many areas: artificial muscles, chemical sensors, visual displays, the moving parts of robots, and batteries.
“The value of electrically active paper is that it’s lightweight and has a high deflection [movement] at low voltage” compared to traditional electroactive polymers, says Sang Choi, senior research scientist at the NASA Langley Research Center. When a small voltage is applied to Kim’s paper, it can move a relatively large distance; for instance, in experiments, the tip of a 30-millimeter-long strip of electroactive paper was displaced 4.2 millimeters. Indeed, the strength of the electric field required to move the tip of the paper to its maximum displacement is one to two orders of magnitude less than is required by other electroactive polymers. And the paper can change shape quickly, moving back and forth as fast as once every 0.06 seconds.
NASA’s Choi is interested in Kim’s material because, compared with conventional piezoelectrics and other electroactive polymers, it is very lightweight and requires very little power. Together, Choi and Kim are designing a small flying vehicle with cellulose wings powered by ambient microwaves. Choi says NASA expects such robots to play an important role in its long-term exploratory missions. For example, small robots with moving parts made of paper or other materials might fly low over the Martian surface to monitor its topology. Still, it’s not clear that cellulose can withstand the extreme conditions in outer space.
The cellulose films that Kim has made so far cannot exert much force – a must for robotics applications. So he’s working with Zoubeida Ounaies, assistant professor of aerospace engineering at Texas A&M University, to strengthen this “smart” cellulose. Ounaies adds carbon nanotubes, prized for their high electrical conductivity and strength, to dissolved cellulose. The mixture is still under study, but the idea is that films of cellulose strands intimately tangled with carbon nanotubes can exert more force than pure cellulose films.
Cellulose is cheap and readily available – Kim’s film can even be made by treating commercially available paper. By comparison, the most commonly used electrically active polymer, polyaniline, costs $68 per gram, says Victoria Finkenstadt, a research chemist at the USDA Agricultural Research Service. Although the robustness and strength of cellulose have yet to be demonstrated, it may also prove to be a good material for the artificial muscles used in robotics, says Finkenstadt.
“These materials may give us [robot] locomotion we’ve never dreamed of,” says Kwang J. Kim, associate professor of mechanical engineering at the University of Nevada in Reno (who was not involved with the cellulose research). But Kim says the field of electrically active polymers is still young, and researchers are still developing applications. “In a few more years interesting technologies will be coming out,” he predicts.